This invention relates to electrical power control apparatus and methods.
The invention is more particularly, but not exclusively, concerned with controlling the power of electrosurgery apparatus.
Electrosurgery, such as diathermy, units are often used during surgery such as to cut tissue and coagulate blood vessels. Radio frequency current is supplied via a hand-held electrode that is applied to the surgical site and a return path is provided from the patient by means of a large area electrode fixed to some other part of the patient's body. In order to provide safe efficient operation it is necessary to control the power delivered to the patient as the load impedance changes. In this respect, the load impedance will vary according to the area of the hand-held electrode that contacts the tissue and according to the nature of the tissue contacted. The power level also needs to be adjustable by the surgeon according to the surgical process in which the apparatus is being used.
At present, this control of power is achieved by means of feedback from the output, by analog processing and by pre-set current and voltage limits. The present techniques suffer from the difficulty that it is not readily possible to vary the power curves of the apparatus, that is, the way in which power varies with impedance, according to different functions that the apparatus may need to perform and according to the front panel settings made by the user. It may be desirable to be able to vary the shape of the power curves at different power level settings but this is not readily achievable with existing apparatus. It is also difficult to compensate for non-linearity in the analog processing which would give rise to deviations from the desired power curves.